Phagocytosis is an important function in macrophages, and is essential for maintaining their tumoricidal and bactericidal functions. It is known that many reactive oxygen species (superoxide, nitric oxide (NO) + peroxynitride) are formed within and surrounding the ingested phagosome so as to destroy the ingested particle within. We have shown that addition of zymosan (in yeast extract) to macrophage stimulates a typical oxidative burst - with an increase in oxygen utilization as the zymosan/cell ratio is increased, oxygen consumption by these cells is inhibited. At the highest concentration of zymosan/cell (approx. 40) oxygen con sumption was very low (as shown by the rate of change in the linewidth of 15 N PDT). Experiments are underway to investigate whether production of nitric ox ide, in addition to having a toxic effect on the ingested particle/species, may be toxic to the cell if overproduced. Two important linesof investigation are apparent; to study the extent of binding of NO to ferrous protein complexes with in the cells (such as those enzymes essential to the respiratory chain), and als o to monitor the intrinsic metal ion EPR signal obtained from zymosan itself as it interacts with macrophages.